1. Field of the Invention
This invention relates to the field of current limiting circuits, particularly circuits used to limit the drive current delivered to the control input of a voltage regulator's pass transistor.
2. Description of the Related Art
A conventional series pass voltage regulator is shown in FIG. 1a. A supply voltage V.sub.in is connected to the emitter 10 of a "pass transistor" 12, typically a pnp bipolar transistor, and an output voltage V.sub.out is taken at the transistor's collector 14. The output voltage is regulated by controlling pass transistor 12 via its base terminal 16. Regulation is accomplished with a feedback loop: the output voltage is fed back to the inverting input 18 of an error amplifier 20, typically an operational transconductance amplifier (OTA), usually via a voltage divider 22. A voltage reference V.sub.ref is connected to the non-inverting input 24 of the amplifier. The amplifier's output is connected to the control input 26 of an output drive transistor 28, whose current circuit is connected to the pass transistor's control input 16.
In operation, error amplifier 20 produces the output necessary to make the voltage at its inputs 18 and 24 equal. Increasing the drive current to output drive transistor 28 increases its collector current i.sub.c, which in turn increases the current flow through pass transistor 12 and raises output voltage V.sub.out.
A regulator such as that shown in FIG. 1 is commonly fabricated as an integrated circuit (I.C.). A problem arises with such an integrated regulator as a result of the unpredictability of the respective "betas" (.beta.) of the transistors in the regulation loop. The OTA 20 has an output transistor 30 having a beta of .beta.1, output drive transistor 28 has a beta of .beta.2, and the pass transistor has a beta of .beta.3. Manufacturing tolerances make it difficult to attain a particular beta value for a particular transistor; rather, a range of possible beta values is typically all that can be predicted. To insure that the regulator can deliver its rated output voltage and current, the regulation loop is usually designed based on "worst case" beta values, resulting in transistors that are likely to be oversized. If V.sub.out drops below its rated value, because the regulator output is short-circuited, for example, the regulator loop will attempt to force V.sub.out back up. However, if .beta.1 is not at its "worst case" value, the drive into output drive transistor 28 may be higher than desired. This high drive current can be compounded by a higher-than-expected .beta.2, resulting in a very high i.sub.c at the pass transistor's base 16. A higher-than-expected .beta.3 compounds the problem further, and can result in a current through pass transistor 12 that is high enough to damage transistor 12 and associated components.
A simplified schematic of a "low drop-out" (LDO) series pass regulator, described in U.S. Pat. No. 5,631,598 to Miranda et. al and assigned to the present assignee, is shown in FIG. 1b. The signals connected to the inputs 18 and 24 of OTA 20 are reversed, and an inverting stage 50 is interposed between the OTA's output and output drive transistor 28. The phase inversion provided by inverting stage 50 permits the connection of a frequency compensation capacitor C.sub.c between the output of OTA 20 and the V.sub.out terminal. This regulator, however, also suffers from the problem discussed above: because the regulator must be designed to accommodate uncertain "worst case" beta values, the potential for overdriving and damaging the pass transistor is unacceptably high.